1. Field of the Invention
The present invention relates to an image processing apparatus that works in response to a frequency diffused clock as a reference to process input image signals, and more specifically pertains to a technique of analyzing specification of an input image signal.
2. Description of the Related Art
Noises arising in a variety of electronic apparatuses may have undesirable electromagnetic interference (EMI) against operations of other electronic apparatuses. There are accordingly regulations set with regard to the noise arising in electronic apparatuses (hereafter referred to as the ‘EMI noise’).
Makers that develop and manufacture the electronic apparatuses have made efforts to reduce the EMI noise arising in the electronic apparatuses and make the electronic apparatuses satisfy the regulations of the EMI noise.
An electronic circuit in the electronic apparatus generally works in response to one or multiple clocks as a reference. FIG. 7 shows a frequency spectrum of a clock. As shown by a curve of solid line, the frequency spectrum of the clock has peaks of an amplitude at an oscillation frequency (fundamental wave: f1) of the clock and frequencies of its harmonic waves (f2, f3, . . . ). The frequency spectrum of the EMI noise arising in the electronic apparatus accordingly has peaks of the amplitude at the oscillation frequency (f1) of the clock and the frequencies of its harmonic waves (f2, f3, . . . ). For reduction of the EMI noise, it is required to reduce the peaks of the amplitude at the frequencies of the fundamental wave of the clock and its harmonic waves. One applicable technique for the reduction varies the oscillation frequency of the clock to diffuse the frequency spectrum as shown by a curve of broken line in FIG. 7 and thereby reduce the peaks of the amplitude at the frequencies of the fundamental wave (f1) of the clock and its harmonic waves (f2, f3, . . . ) in the frequency spectrum. In the description hereafter, the technique of varying the oscillation frequency of the clock to diffuse the frequency spectrum is referred to as ‘frequency diffusion’. The variation in oscillation frequency of the clock is referred to as the ‘diffusion quantity’. The greater diffusion quantity generally results in reducing the EMI noise by a greater degree.
An image display apparatus is one of the electronic apparatuses that carry out diverse series of processing synchronously with a clock as a standard of operations. Reduction of the EMI noise is thus demanded in the image display apparatus. The prior art technique for reducing the EMI noise due to the frequency diffusion in the image display apparatus, however, has a problem discussed below.
There are a diversity of specifications of the image signal based on various conditions, such as the type of the apparatus outputting the image signal (for example, a computer, a DVD player, or a video recorder) and the resolution of the image expressed by the image signal. The image signal generally does not include information that directly represents the specification of the image signal. In order to allow display of images expressed by image signals of diverse specifications, the image display apparatus analyzes the input image signal and processes the input image signal according to the analyzed specification of the image signal.
In the image display apparatus, a typical procedure of analyzing the image signal measures a synchronizing signal period, a synchronizing signal cycle, and polarity of the synchronizing signal as the synchronizing signal characteristic values and reads the specification of the image signal corresponding to the observed synchronizing signal characteristic values from a database, which has been prepared in advance. The synchronizing signal cycle and the synchronizing signal period are measured, for example, by counting measurement clocks corresponding to the synchronizing signal cycle and the synchronizing signal period. The measurement clock is typically generated in response to a system clock, which is a standard of operations in the image display apparatus.
In the image display apparatus, frequency diffusion of the system clock as the standard of operations in order to reduce the EMI noise results in frequency diffusion of the measurement clock, which is generated from the system clock. The frequency diffusion of the measurement clock may cause a variation in observed synchronizing signal characteristic value with a variation in oscillation frequency. This leads to wrong analysis that the specification of the image signal is changed, while the specification of the image signal is actually unchanged. It is accordingly difficult to reduce the EMI noise due to the frequency diffusion in the image display apparatus.
The drawback of the prior art technique discussed above is not restricted in the image display apparatus, but commonly arises in image processing apparatuses that work in response to a clock as a standard of operations and process image signals.